The specification for World System Teletext (WST) is set out in the document "World System Teletext and Data Broadcasting System"--Technical Specification, published December 1987 by the United Kingdom Department of Trade and Industry.
One of the major factors which distinguishes WST from other teletext systems is its direct mapping between transmission codes and display codes. Teletext data is transmitted in the form of transmission packets which comprise 40 data bytes. Each data packet corresponds to a row of a teletext display so that the 40 data bytes of each packet are directly mapped into 40 character or attribute positions on the corresponding display row. Typically, a teletext page is formed by 24 display rows, so 24 data packets are required to transmit each teletext page. This "Fixed Format" principle leads to simple and cheap decoders which are robust in the presence of reception errors. The Fixed Format principle is generally considered to be one of the main reasons why WST has been so successful, it being used presently in some 35 countries worldwide.
In WST a range of spacing attributes is specified as set out in FIG. 5 of the aforementioned WST Technical Specification and enables, for example, a color change to be introduced in a teletext display. A limitation of the WST Fixed Format principle is that such spacing attributes are transmitted in the same way that actual characters are transmitted and cause a blank space to be displayed wherever they occur in the display rows. It follows from this that the spacing attributes can only be placed in the natural space between words, otherwise they will cause an unwanted space to be inserted in the text. Usually this does not cause a problem but nevertheless it imposes a restriction which some editors might consider to be a problem since, for example, it is not possible in basic (level 1) WST to display every letter of a word in a different colour. Higher level versions of WST (level 2 and above) are provided with coding extensions which make non-spacing attributes possible, but the implementation of these coding extensions imply additional hardware and software in the decoder and require more time in which to transmit the additional codes.
In ideographic (e.g. Chinese) writing systems, each ideogram represents an idea or concept, and not a sound as is the case with Latin based systems, and consequently there are no spaces within a sentence where an attribute could be placed naturally. Higher level (level 2) coding extensions could be applied to ideographic teletext but this would involve an increase in cost for even the simplest ideographic decoder.